This invention relates generally to the control of motors and, more particularly, to a method of limiting to a safe level the power output of a variable speed motor operating under variable load conditions.
The use of variable speed motors is now being made for various applications. In order to vary the speeds as desired, various control techniques have been employed. One approach is that of pulse width modulation wherein the voltage is controlled by varying the duration and/or width of the output pulses. Pulse width modulation can be used, for example, with inverter controls or with electrically commutated motors (ECM's).
In the application of motor controls, it is common to limit the speed of the motor to a safe operating level as determined by the particular design specifications. Even when operating within this limit, a motor or its associated electronics may become overstressed if its power output exceeds a certain level for a given load on the motor. For example, where a motor is used to drive a fan for the circulation of air from a furnace, the speed of the motor is controlled such that the power output will not normally exceed a safe level. However, where a high volume of air delivery is called for under nonstandard conditions such as static pressures in excess of 0.5 inches of water, the associated higher torque requirements may cause the safe power output level to be exceeded. This may occur, for example, where the air filter becomes dirty or the ductwork is overly restrictive because of improper sizing or because of too many dampers being closed off.
In Patent Application Ser. No. 809,466, filed Dec. 16, 1985 by the common assignee of the present invention, incorporated herein by reference a technique is described for calibrating a variable speed motor. As part of this technique, a motor fan combination is tested to empirically determine the relationship between the speed of the motor and the volume flow of air being delivered by the fan. This relationship is then used, in combination with the fan law equations and a measured torque output of the motor, to calculate the desired motor speed to obtain a desired air delivery volume. However, such a motor and control system is susceptible to the problem mentioned above relating to excessive power conditions.
It is therefore an object of the present invention to provide a means of protecting a variable speed motor from exceeding a safe level of power output.
Another object of the present invention is the provision for controlling the power output of a variable speed motor to which an unusually high load may be applied.
Yet another object of the present invention is the provision for controlling the output of a variable speed motor without the use of special equipment and apparatus.
Still another object of the present invention is the provision for an improved motor control system that is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.